Chapter 13 Electromagnetism

WARNING Strong magnets will be used in the room. Keep these magnets away from iPods and laptops (especially my laptop)!

(13.1) Magnetism

(13.1) What is Magnetism? Magnets have been known for centuries Ancient Greeks used a stone substance called “magnetite” Later, stones of magnetite called “lodestones” were used in navigation

Magnetic Fields The ends of a magnet are where the magnetic effect is the strongest These are called “poles” – N and S

Pairs Poles of a magnet always come in pairs! If you cut a magnet in half, you get 2 magnets! S N S N S N

Like repels like… Opposites attract!

Magnetic Fields magnetic field lines are drawn to represent the magnetic field produced by certain arrangements of bar magnets

Magnetic Fields

(13.2) Magnetic Materials

The Domain Theory Atoms have magnetic properties due to the spin of their electrons Groups of atoms join so that their magnetic fields are all going in the same direction These areas of atoms are called “domains”

Domain Theory - The Effects An unmagnetized substance looks like this… While a magnetized substance looks like this…

Breaking a Magnet Drop it Heat it This causes the domains to become random again!

Direction A compass needle is a magnet

Direction of Field Needle of a magnetic compass will follow the magnetic field lines, with the north end showing the direction of the field.

Direction of Field Needle of a magnetic compass will follow the magnetic field lines

Other Magnet Shapes Magnetic field lines around horseshoe and disk magnets are closest at the poles

The Earth is a Magnet Earth exerts magnetic forces and is surrounded by a magnetic field that is strongest near the North and South magnetic poles

Giant Magnet The nickel iron core of the earth gives the Earth a magnetic field much like a bar magnet.

North Pole? The needle of a compass always points toward the magnetic south pole. We call this direction “North” remember, opposites attract

Can we see it? Aurora Borealis

(13.3) Oersted’s Discovery

Principle of Electromagnetism Whenever an electric current moves through a conductor a magnetic field is created around the conductor.

Oersted’s Discovery A compass needle below a wire (A) points north when there is no current, (B) moves at right angles when a current flows one way, and (C) moves at right angles in the opposite direction when the current is reversed.

An electric current produces a magnetic field. Use mounted wire with platform from 2B lab. Magnetic Field

Right-Hand Rule for a Conductor If a straight conductor is held in the right hand with the right thumb pointing in the direction of the electric current, the curled fingers will point in the direction of the magnetic field lines. *** conventional current***! Figure 3 (p479)

How Do we Illustrate That? Figure 4 (p480) Models of the magnetic field of a straight conductor (a) Imagine the X as being the tail of an arrow moving away from you. (b) Imagine the dot as being the tip of an arrow facing you.

Let’s Practice! p. 481 #1 Indicate the direction of electric current in each wire.

Let’s Practice! p. 481 #2 Draw magnetic field lines around each.

Homework Read textbook p470-488 Do p475 #2, 4 p482 #2 (extra # 1,3)